1. Field of the Invention
The present invention relates to a switching circuit and a semiconductor module.
2. Description of the Related Art
A high-frequency switch is a switch for switching a transmission path for high-frequency signals. For example, in a wireless communication device, such as a cellular phone or a wireless local area network (LAN), a high-frequency switch is used to switch a frequency band or to switch a transmission path between that for a transmission signal and that for a reception signal.
One example of a single pole double throw (SPDT) switching circuit is disclosed in Japanese Unexamined Patent Application Publication No. 9-107203. This switching circuit aims to have high isolation at a desired frequency. This switching circuit switches a signal transmission path between a first transmission path for use in transmitting a signal from an input/output terminal to a reception terminal and a second transmission path for use in transmitting a signal from a transmission terminal to the input/output terminal. This switching circuit includes an inductor disposed between the transmission terminal and the reception terminal.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 9-107203, a parasitic capacitance in a field-effect transistor (FET) and the inductor constitute a resonant circuit. A resonant frequency of the resonant circuit is set at a frequency for use. Thus it is expected that high isolation is achieved at that frequency for use.
However, an increase in isolation of the switching circuit expands isolation deviation in a predetermined frequency band including that frequency for use. The resonant frequency varies depending on factors, such as variations in inductance value or variations in parasitic capacitance of the FET. When the isolation deviation is large, a problem may arise in that because of the variations in resonant frequency, there are large variations in isolation characteristics among a plurality of switching circuits having the same configuration.